Ring laser angular rate sensors, sometimes referred to as ring laser gyros, are well known in the art. A detailed description may be found in the "Background of the Invention" of U.S. Pat. No. 4,597,667, which is incorporated herein by reference. Briefly, such sensors include a ring laser supported in a block having plurality of gas containing tunnels. At the intersection of the tunnels are mirrors to define a closed-loop optical path which is traveled by counter-propagating laser beams therein. Practical embodiments of ring laser angular sensors usually include a path length controller. The purpose of the path length controller is to maintain a constant path length. Maintaining a constant path length avoids false rotation errors in the usual sensor output. The path length controller function is usually provided by one of the mirrors being attached to a piezoelectric transducer which controls translational movement of the mirror to effect the laser beam path length in response to a drive signal provided through a transducer drive amplifier.
One technique for maintaining a constant path length is detecting the intensity of one or both of the laser beams and controlling the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071 which issued May 1, 1979 to T. J. Podgorski, and assigned to the assignee of the present invention illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T. J. Podgorski, also assigned to the assignee of the present invention, U.S. Pat No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H. G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat No. 4,320,974, which issued on Mar. 23, 1982 to Bo H. G. Ljung, and is also incorporated herein by reference.
In path length control systems of the prior art, the path length control finds mirror positioning for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency, as indicated by a spectral line, of the lasing gas. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power. The properly designed ring laser has a maximum power at traverse modes commonly referred to as "axial" or "on axis" modes. There are many longitudinal on-axis modes of operation which satisfy the intended operating condition. Unfortunately, there are other subsidiary parasitic or secondary maximums between different on-axis modes. These parasitic maximums are sometimes referred to as "off-axis" modes. The corresponding laser power at off-axis modes is less than when the laser is operating at the on-axis modes.
Path length control systems of the prior art are intended to operate at the maximum power or on-axis mode so that laser sensor performance is optimum. Operation of the laser sensor at off-axis modes can lead to an introduction of sensor rotation and performance errors. It is sometimes necessary during operation of a ring laser gyro to reset the transducer drive voltage to the center of the transducer drive amplifier's drive range. This reset is usually accomplished by shorting out a capacitor in the integrator within the path length control circuit loop. The path length control circuit control loop will usually drive the mirror transducer so as to acquire the closest laser intensity peak to the reset voltage as is shown graphically in FIG. 2.
Referring now to FIG. 2, a graphical representation of laser beam intensity verses transducer drive voltage is shown. Typically, as is shown in FIG. 2, the laser intensity peaks, for example, 201 and 202 occur near the reset drive voltage and therefore the laser intensity at the reset drive voltage is shown at point 205. In such cases, the path length control circuit will drive the linear transducer 14 so as to acquire the closest laser intensity peak to the reset voltage. In this example, peak 202 will be acquired.
Referring now to FIG. 3, a graphical representation of laser beam intensity verses transducer drive voltage is shown wherein the reset command puts the ring laser gyro in an indeterminant state between laser intensity peaks. In this example, the reset drive voltage intersects the laser intensity at a point 305 which is close to or at the midpoint between two laser intensity peaks 301 and 302. In the prior art, the path length control circuit loop cannot determine which laser intensity peak to acquire and consequently can stay in this indeterminant state between peaks for an unexceptably long duration until either noise or integrator offsets finally allow the loop to determine which peak is closer and, to thereby acquire that peak. The reset acceleration circuit of the present invention solves this problem.